Alkylhalosilanes and arylhalosilanes are valuable precursors to silicones and organofunctional silanes that are used in a broad range of industries. Methykhlorosilanes and phenylchlorosilanes are particularly valuable and are the most commonly manufactured products of these classes. The primary commercial method to prepare alkylhalosilanes and arylhalosilanes is through the Rochow-Müller Direct Process (also called Direct Synthesis and Direct Reaction), in which copper-activated silicon is reacted with the corresponding organohalide in a gas-solid or slurry-phase reactor. Gaseous products and unreacted organohalide, along with fine particulates, are continuously removed from the reactor. Hot effluent exiting from the reactor comprises a mixture of copper, metal halides, silicon, silicides, carbon, gaseous organohalide, organohalosilanes, organohalodisilanes, carbosilanes and hydrocarbons. Typically this mixture is first subjected to gas-solid separation in cyclones and filters. Then the gaseous mixture and ultrafine solids are condensed in a settler or slurry tank from which the organohalide, organohalosilanes, hydrocarbons and a portion of organohalodisilanes and carbosilanes are evaporated and sent to fractional distillation to recover the organohalosilane monomers. The solids accumulated in the settler along with the less volatile silicon-containing compounds are purged periodically and sent to waste disposal or to secondary treatment such as catalytic hydrochlorination. Organohalodisilanes and carbosilanes left in the post-distillation residues are also fed to hydrochlorination.
Organohalodisilanes, organohalopolysilanes and carbosilanes, related siloxanes and hydrocarbons, either in the post-distillation residues or in the slurry purged from the reactor, boil above orgaohalosilane monomers. Collectively they are referred to as Direct Process Residue (DPR). The terms, higher boilers, high-boiling residue and disilane fraction, are also used interchangeably with DPR. DPR can account for 1 to 10 weight percent of the Direct Synthesis product mixture and, owing to the considerable accompanying tonnage, about 1 to 8 percent of the total raw material cost of the Rochow-Müller Process. Accordingly, there have been many attempts to recover organohalosilane monomers and other values from DPR through cleavage, redistribution and disproportionation processes.
Cleavage is the term used to describe the process whereby disilanes, trisilanes, polysilanes and carbosilanes are reacted to produce monomeric silanes. Hydrochlorination and hydrogenolysis are examples of cleavage processes. Redistribution is the rearrangement of groups bonded to silicon atoms such that new molecules are produced during the reaction. For example, in the equation shown below, the compounds of the class R12SiX2 are formed by redistribution of R13SiX and R1SiX3. The reverse reaction, whereby R12SiX2 is converted to the original reactants is called disproportionation.R13SiX+R1SiX3⇄2R12SiX2  (1)
Illustratively, in the case of the secondary treatment of the DPR from the Direct Synthesis of methylchlorosilanes, the literature have disclosed the following: catalytic hydrochlorination as is disclosed in U.S. Pat. No. 2,598,435; U.S. Pat. No. 2,681,355; U.S. Pat. No. 2,709,176; U.S. Pat. No. 2,842,580; U.S. Pat. No. 3,432,537, U.S. Pat. No. 5,627,298, EP 861844; and H. Matsumoto, et al., Bulletin Chemical Society Japan, vol 51 (1978) 1913-1914; catalytic hydrochlorination with immobilized tertiary amine catalysts as disclosed in DE 4,207,299; thermal hydrochlorination as is disclosed in EP 1533315 and U.S. Pat. No. 5,292,912; and by K. Shiina, et al., Chemical Abstracts., vol 53 (1957) 17889b; catalytic hydrogenolysis as is disclosed in U.S. Pat. No. 2,787,627; U.S. Pat. No. 3,639,105; U.S. Pat. No. 4,070,071; U.S. Pat. No. 4,059,608; U.S. Pat. No. 5,292,909; U.S. Pat. No. 5,326,896 as well as in K. M. Lewis, 203rd ACS National Meeting, San Francisco, April 1992, Abstract INOR 52; and A. Taketa, et al., Chemical Abstracts, vol 53 (1957) 17888i; catalytic redistribution/disproportionation with Lewis Acids as is disclosed in U.S. Pat. No. 4,393,229; U.S. Pat. No. 4,552,973; U.S. Pat. No. 4,888,435 as well as in J. Urenovitch, et al, J. Amer. Chem. Soc., vol 83 (1963) 3372-3375, ibid. 5563-5564; H. Matsumoto, et al., J. Organometallic Chemistry, vol 142 (1977) pp 149-153; Sakurai, et al., Tetrahedron Letters, #45 (1966) 5493-5497; and Ishikawa, et al., J. Organometallic Chemistry, vol 23 (1970) 63-69; Lewis Acid catalyzed catalytic redistribution of Direct Process Residue with methylchlorosilane monomers, including the lower boiling fraction (boiling point <43° C.) from the Direct Process, as has been disclosed in U.S. Pat. No. 4,393,229; DE 3,208,829; DE 3,436,381; DE 3,410,644; US 2005/0113592 A1 and by Zhang, et al., Res. Chem. Intermed., vol 33 (2007) pp 613-622; catalytic redistribution/disproportionation with Lewis Bases as is disclosed in U.S. Pat. No. 4,298,559; U.S. Pat. No. 5,416,232; Fr. Pat. 2,427,302 as well as in R. Trandwell, et al., J. Inorg. Nucl. Chem., vol 40 (1978) 1405-1410; C. Garcia-Escomel, et al., Inorg. Chim. Acta., vol 350 (2003) 407-413 and R. Calas, et al., J. Organometallic Chemistry, vol 71 (1974) 371-376.
One limitation of the prior art processes is the inability of these processes to convert highly methylated chlorodisilanes, such as (CH3)3SiSi(CH3)2Cl and Cl(CH3)2SiSi(CH3)2Cl, or carbosilanes such as Cl2CH3Si—CH2—Si(CH3)2Cl to organohalosilane monomers. Illustratively, Trandell et al., (J. Inorg. Nucl. Chem., 40 (1978) 1305-1308) reported that Cl(CH3)2SiSi(CH3)2Cl did not disproportionate in the presence of trimethylamine even when heated to 65° C. for four months and 100° C. for two months. Garcia-Escomel, et al., (Inorg. Chim, Acta, Vol 350 (2003) 407-413) state that Cl(CH3)2SiSi(CH3)2Cl was unreactive when treated with a variety of phosphines, phosphites, phosphine oxides and tetralkylammonium halide Lewis bases at 140-150° C. According to Baney, et al., (Organometallics, 2 (1983) 859-864) Cl(CH3)2SiSi(CH3)2Cl remains unreacted when heated with tetrabutyl phosphonium chloride up to 150° C. Herzog, et al. (J. Organometallic Chem. 507 (1996) 221-229) employed higher, unspecified temperatures and N-methylimidazole as catalyst and observed the formation of a white solid, characterized as (CH3)2SiCl2 complexed with two molecules of the catalyst, and tri- and tetra-silanes.
Recently, there are disclosures that are said to produce monomeric silanes from highly methylated chlorodisilanes. For example, JP A 54-9228 discloses the hydrochlorination of Cl(CH3)2SiSi(CH3)2Cl with [(C6H5)3P]4Pd as catalyst to produce (CH3)2SiHCl. For the same purpose, U.S. Pat. No. 5,502,230 discloses the use of a catalyst composition consisting of Pd(0) or Pt(0) and an additive chosen from a tertiary amine, carboxylic amide, alkylurea, tertiary phosphine, phosphoric amide, quaternary ammonium halide or quaternary phosphonium halide. U.S. Pat. No. 7,655,812 discloses a method of preparing (CH3)2SiHCl via hydrochlorination of Cl(CH3)2SiSi(CH3)2Cl comprising the use of Pd(0), a tertiary amine and a tertiary phosphine in which at least one of the hydrocarbyl groups is a functionalized aryl group. However, all these processes require the use of expensive noble metals in the catalyst compositions thus making them too expensive to be commercially practicable.
Furthermore, even for conventionally cleavable DPR, the monomers produced by the prior art processes tend to contain more CH3SiCl3 monomer than otherwise would be desirable. It is generally agreed that the organohalosilane monomers of general formula R1SiX3 are less valuable than those of general formula, R12SiX2, R13SiX, R1SiHX2 and R12SiHX. In the case of methylchlorosilanes, the compounds can be ranked in value based on selling prices of commercial quantities or of smaller amounts for laboratory research. Using prices published on the internet or in specialty chemical catalogs, such as Gelest, Inc., the value ranking of the methylchloro-silane monomers is (CH3)2SiHCl>CH3SiHCl2>(CH3)3SiCl>(CH3)2SiCl2>CH3SiCl3. In current commercial practice, the high-boiling liquid residue from the methylchlorosilane Direct Process is reacted with HCl in the presence of a tertiary amine catalyst, such as tri(n-butyl)amine, at temperatures in the range of 140° C. to 180° C. Unfortunately, the monomer mixture produced by this commercial process is enriched in less valuable CH3SiCl3 relative to other more valuable monomers and the gravimetric ratio, (CH3SiCl3/(CH3)2SiCl2), is typically greater than 1.0.
Accordingly, an objective of the present invention is the provision of an enhanced process for preparing organohalosilane monomers from conventionally cleavable Direct Process Residue which does not involve the use of expensive noble catalysts, which provides reduced R1SiX3 and increased R12SiHX, R1SiHX2 and R12SiX2 compared to conventional commercial processes such as tertiary amine catalyzed hydrochlorination, and which is easily conducted at moderate temperatures and relatively short reaction times.